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Weldability of stainless steels
The document discusses the weldability of various stainless steel types, including austenitic, ferritic, and martensitic stainless steels. It provides information on their typical compositions and applications. It also describes various welding techniques that can be used and issues that may occur during welding like sensitization, sigma phase formation, and hydrogen cracking. Prevention methods are outlined like using stabilizers, annealing treatments, and controlling cooling rates and heat inputs during welding.
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Weldability of stainless steels
- 1. NAME : K.JOTHIKARTHICK DEPT. : WELDING METALLUGY REG NO : 1763009
- 2. Weldability of stainlesssteels • There are four types such as •Austenitic stainless steel •Ferritic stainless steel •Martensitic stainless steel
- 3. WELDABILITY OF AUTENITICSTAINLESS STEEL • It has the following composition • C 0.03 to 0.25% Mn 2 to 10% • Si 1 to 2% Cr 16 to26% • Ni 3.5 to 22% P and S 0.008% Cb and Ta • Mo and Ti in somecase fe balance • They posses high corrosion resistance of all the Stainless steels. • They posses great strength and ductility and non magnetic and so on. • Application • Aircraft industry • Austenitic – process plants, cryogenics, medium corrosive applications, used over wide temperature range • Chemical processing (heat exchanger) • Dairy industry( milk cans)
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- 5. Welding techniques • Oxy– acetylene welding • Arc welding • SMAW • GMAW • GTAW • SAW • PAW • Resistance welding • Flash butt welding • Projection welding • Seam welding • Spot welding • Percussion welding • Brazing
- 6. Welding procedure • Thermalconductivity is low about 50% • Melting point is low • Thermal expansion is about 50% lower • Electrical resistance is about 6 times greater • The first 4 factor combine require for low welding current
- 7. Problems associated withaustenitic SS • Ferrite and sigma phase formation • Carbide precipitation • Stress corrosion cracks • Ferrite and sigma phase formation This problem is corrected by that electrode that deposit weld metal containing 3 to 5% of ferrite • Sigma phase can be redissolved by heating above 900 degree celsius for short time Austenite stabilisers, Nieq Ferrite stabilisers, Creq • Carbon, Nickel, Nitrogen, • Chromium, Molybdenum,v Manganese, Copper ,Tungsten, Niobium, Silicon
- 8. Carbide precipitation • Thermaltreatment(845 t0 425 with fast cooling) • Use of low carbon SS • Stabilizer • Columbium • Niobium • Titanium • tantalum Stress corrosion cracking(SCC) • Annealing treatment • Remove chloride and oxygen
- 9. Weldability of Ferriticstainless steels • It has following composition • C 0.08 to 1.5% Si 1% • Mn 1 to 2% Cr 11 to 27% • S 0.04% P 0.04% • Fe balance • They possess magnetic and good ductility • It is more corrosion resist than martensitic steels. • Application • Lining petroleum industry • Heating element for furnace • Screws and fitting
- 10. Ferritic SS • Sensitiveto grain growth – loss of toughness, embrittlement – limited to 250°C service temperature – can lead to cracking in weld or HAZ of highly restrained joints and thick section material. • Hydrogen cracking • Filler metals: • – matching, austenitic or nickel alloy
- 11. Welding techniques • GMAW •SAW • GTAW • SMAW • Resistance welding • Flash butt welding • Projection welding • Seam welding • Spot welding • Percussion welding
- 12. • The weldingquality of ferritic Stainless steels is poorest of AISI types • Welding heat a zone in the base metal above critical temp 955 and cause rapid grain growth of ferrite • Excess grain growth avoid by lower heat inputs • Adding Al,V,Zr to suppress the grain growth during welding • Intergranular corrosion avoided by annaling 650 to 815 about 10 min to 60mine
- 13. Preheating • A preheating150 or higher to minimize residual stress • Thin section not require • Thick section require for preheating PWHT(post weld heat treatment) • Reduce harness and residual stress
- 14. Weldability of martensiticStainless steels • It has following composition • C 0.15 to 3% Si 1.8% • Mn 3.5% Cr 11 to 18% • Fe balance Ni 1.25% • They possess hardness and ductility and magnetic and so on. • It is best thermal conductivity of SS. • It uses of martensitic Stainless steels • Pump and valve • Surgical instrument • Rules an tape
- 15. Welding techniques • SMAW •GTAW • GMAW • SAW • SPOT Welding • Flash welding • Friction welding • EBW
- 16. • Welding Problems •Hydrogen cracking • Normally matching consumables. • High hardness in the HAZ makes this type of stainless steel very prone to hydrogen cracking . • The risk of cracking increasing with the carbon content. • Above C .15 to 3% not suitable for welding. • Underbead crack • Take precautions to minimize • Preheating • PWHT
- 17. Sensitization • Sensitization refersto the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the alloy to be susceptible to intergranular corrosion. • This results in susceptibility to intergranular corrosion. • austentitic stainless steel may become sensitised if they are heat treated or used at temperature in range 500-800c. the heat affected zones of welds may also be sensitised in some circumstance
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- 19. CAUSES • More amountof carbon content • Slow rate of cooling from 425 to 845 so it is also called sensitising temperature range. • Time ,temperature , chemical composition play a vital role in sensitisation.
- 20. Prevention • Fast coolingrequires because the carbon does not react with chromium. • Stabilizer • Columbium • Niobium • Titanium • Tantalum • Both base materials and fill rod have low carbon content.
- 21. Sigma embrittlement • Sigmaphase is a non-magnetic composed mainly of iron and chromium which forms in ferritic and austenitic stainless steels during exposure at 560º- 980ºC . • Many stainless steels and other iron-chromium alloys are susceptible grain boundary phenomenon known as sigma-phase embrittlement. • Alloy elements such as molybdenum, titanium and silicon promote the formation of sigma phase • Molybdenum promotes sigma-phase formation much more effectively than chromium, particularly at temperatures around 900°C (1650°C)
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- 23. causes • Loss ofductility and toughness • Corrosion resistance resulting in cracking • Failure of components • Especially those subjected to impact loads or excessive stress
- 24. PREVENTION • Nitrogen andcarbon reduce as it tendency to form. • Annealing treatment.
- 25. Mode of solidification •Solidification solidification, also known as freezing, is a phase change of matter that results in the production of a solid. Generally, this occurs when the temperature of a liquid is lowered below its freezing point • There are four modes • Planar • Cellular • Columnar Dendrite • Equiaxed dendrite
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- 28. Cellular and columnardendrite
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